<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      input {
        color: #999;
        outline: none;
      }
    </style>
  </head>
  <body>
    <input type="text" value="手机" />
    <script>
      //onfocus 获得焦点
      var input = document.querySelector("input");
      input.onfocus = function () {
        input.style.color = "#000";
        if (this.value == "手机") {
          input.value = "";
        }
      };
      //onblur 失去焦点
      input.onblur = function () {
        input.style.color = "#999";
        if (this.value == "") {
          input.value = "手机";
        }
      };
    </script>
  </body>
</html>
